There are varieties of pocket warmers, called ”KAIRO” in Japanese sold in Japan, and you can get one at any convenience stores, supermarkets and drug stores.
The major ones are disposal kairo but reusable eco kairo is getting popular since it appeared in the market.
Different sizes are available for disposal kairo, and there are sticker ones which you can stick on your under shirt or something to keep your back warm.
Eco kairo has a metal chip on gel. When you click the metal, the gel gets warmer.
After you use it, you have to put it in boiled water to soften the gel to be reused.
This is JAPAN Style!
You should follow me on Twitter HERE.